Add a plain-text report format

[amic25]

Goal

Add a dependency-free plain-text report for CI environments that disable color.

Scope

• Preserve severity, rule ID, location, explanation, risk, and remediation.
• Register the format in the CLI.
• Add reporter and CLI tests.
• Document one usage example.

Acceptance criteria

• agentguard scan . --format plain emits deterministic text.
• Output contains no ANSI escape sequences.
• Exit thresholds remain unchanged.
• make check passes.